1. Technical Field of the Invention
This invention relates generally to loudspeaker enclosures, and more specifically to location of damping material in loudspeaker enclosures.
2. Background Art
Acoustic transducers are known to cause vibration, flexure, expansion, contraction, and bending modes in the loudspeaker cabinets to which they are coupled. These effects can be directly caused by the physical coupling of the oscillating transducer to a panel of the cabinet—as the motor rapidly and powerfully extends and withdraws the diaphragm assembly, the non-moving transducer components and the cabinet structures to which they are coupled under go an equal-and-opposite reaction type of mechanically induced movement. Furthermore, the oscillation of the diaphragm assembly causes pressurization and rarefaction of the air volume within the cabinet, especially in a sealed cabinet. Low frequency vibrations can cause gross flexure of the cabinet panels, and even the higher frequency vibrations can cause partial flexure or higher order flexure of the panels.
It is desirable to minimize these vibrations, flexures, etc., as they can interfere with ideal operation of the loudspeaker. They can cause output loss, reducing acoustic output above and below flexure resonance. At a panel's resonant frequency and its harmonics, modes of destructive interference between the enclosure and the transducer cancel some amount of acoustic output of the transducer, and modes of constructive interference add and create higher output spikes in the acoustic output.
Various damping materials have been added to loudspeaker cabinets in attempts to reduce such vibration, expansion, and flexing. Some manufacturers have simply laminated a damping material layer onto the interior surfaces of their cabinets; this is known as extensional damping. Others have sandwiched or laminated damping materials between two or more layers of the cabinet panels or walls; this is known as constrained layer damping. Extensional damping and constrained layer damping are designed to absorb vibrations in the panel structures themselves, and are somewhat in contrast to the practice of placing acoustical batting against the panels to absorb vibrations in the enclosed air itself.
Damping materials function by converting the kinetic energy of the moving panels into heat. Previous configurations have not been especially effective in doing so. Very little compression and expansion of the damping material is induced by the vibration, and very little shear is applied to the damping material because of the geometries of the cabinet panels. When a panel flexes, the extensional or constrained damping layer coupled to it undergoes a very small degree of compression or expansion caused by the change in its curvature. It is very inefficient, geometrically, because the induced shear, compression, and expansion are nearly perpendicular to the direction of the panel motion.
Internal bracing is often added to loudspeaker cabinets, to reduce expansion and flexure of the cabinets. Internal braces can divide the cabinet's enclosed air into two or more separate, isolated volumes, if desired. Or, if the internal braces are small enough (meaning that they do not extend completely over the cross-sectional area of the air volume) or are provided with holes, the enclosed air remains a single effective air volume. Internal bracing stiffens the cabinet, shifting the panels' resonance to higher frequencies, but does not change the amount of damping of the enclosure. It changes the frequency but not the amplitude of the vibrational resonance.
What is needed is an improved loudspeaker cabinet with both improved damping and improved structural rigidity.